1. Field of the Invention
This invention generally relates to an insulating spacer and in particular to an insulating spacer for creating a thermally insulating bridge between spaced-apart panes in a multiple glass window unit, for example, to improve the thermal insulation performance of the unit. This invention also relates to methods of making such an insulating spacer.
2. Description of the Related Art
An important consideration in the construction of buildings is energy conservation. In view of the extensive use of glass in such construction, a particular problem is heat loss through glass surfaces. One solution to this problem has been an increased use of insulating glass units comprising basically two or more glass panels separated by a sealed dry air space. Sealed insulating glass units generally require some means of precisely separating the glass panels, such as by spacers.
The spacers currently used are generally tubular channels made entirely of steel, aluminum or some other metal containing a desiccant to adsorb moisture from the space between the glass panels to thus avoid condensation problems and to keep the sealed air space dry. Tubular spacers are commonly roll-formed into the desired profile shape. Steel spacers are generally cheaper and stronger, but aluminum spacers are easier to cut and install. Aluminum also provides lightweight structural integrity, but it is expensive and tends to be a poor thermal performer. Spacers made entirely of plastic also have been used to a limited extent. However, plastic is permeable, which can result in moisture transmission and condensation.
There are certain significant factors that influence the suitability of the spacer, particularly the heat conducting properties and the coefficient of expansion of the material. Since a metal spacer is a much better heat conductor than the surrounding air space, its use leads to the conduction of heat between the inside glass pane and the outside glass pane resulting in heat dissipation, energy loss, moisture condensation, especially on the sill, and other problems. Further, the coefficient of expansion of commonly used spacer materials is much higher than that of glass. Thus, heat conduction results in a differential dimensional change between the glass and the spacer, thereby causing stresses to develop in the glass and in the seal. This can result in damage to and failure of the sealed glass unit, such as by sufficient lengthwise shrinkage of the spacer to cause it to pull away from the sealant.
The most common material commercially used in the manufacture of such spacer units has been metal. Metal has been used because it has a coefficient of expansion similar to that of glass, among other reasons, and because this property is important in the manufacture of such a unit. Any difference in thermal expansion causes problems. This is particularly true in climates that have large changes in temperature. These consequences include cracking of the glass and at least breaking of the seal between the panes of glass.
Some experimentation has been made with all-plastic spacers, particularly nylon, vinyl, polyvinyl chloride, polycarbonate or other extruded plastic spacers, but these units generally have been thin and structurally weak. In fact, these thin, non-metal spacers can bend undesirably and collapse. Furthermore, to date, most thermoplastics have been unacceptable for use as spacers because they give off volatile materials, e.g., plasticizers, which can cloud or fog the interior glass surface. In view of the above-noted drawbacks, such all-plastic spacers generally have been found unsatisfactory.
Therefore, metal has been the generally accepted material even though this material has a number of disadvantages. In particular, the thermal conductivity of metal is considerably higher than that of glass or of the air space between the panes of glass. In a sealed unit, heat from within a building tries to escape in winter, and it takes the path of least resistance. The path of least resistance is around the perimeter of a sealed window unit, where the metal spacer strip is provided. Metal spacers contacting the inner and outer panes of glass act as conductors between the panes and provide an easy path for the transmission of heat from the inside glass panel to the outside panel. As a result, under low temperature conditions in winter, and when the seal fails, for instance, condensation of moisture can occur inside the insulating glass or on the surfaces of the inner glass panel. Also, heat is rapidly lost from around the perimeter of the window, often causing a ten to twenty degree Fahrenheit temperature drop at the perimeter of the window relative to the center thereof. Under extreme conditions in winter, a frost line can occur around the perimeter of the window unit.
The above-noted temperature differential also results in differential shrinkage between the center of the glass pane and the perimeter. Then, stress cracks can develop in the glass or the seal can be broken. When the outside seal breaks down, air can enter the space between the windows carrying water vapor which is deposited inside the panes. Condensation of this moisture causes fogging of the window unit. Many window units tend to fail due to such stress cracks or loss of seal.
Another problem inherent in previous spacer arrangements is that a rigid spacer provides an excellent path for the transmission of sound from the outer panel to the inside panel. This poses a particular problem in high-noise areas such as airports. Other institutions such as hospitals also have a need for low sound transmission glass units.
A still further problem with conventional glass units is related to deflection of the panels under the influence of high winds, traffic noise, or internal pressure changes owing to expansion or contraction of the air mass contained within the glass unit. This action imposes high stresses on the glass panels and can break the seal between the spacer and the glass thus allowing moisture to enter. In extreme cases, the glass panels can break.
The prior art has attempted to overcome the drawbacks noted above by providing composite spacers. For instance, U.S. Pat. No. 4,113,905 discloses a composite foam spacer for separation of double insulated glass panes. The spacer includes a thin extruded metal or plastic core and a relatively thick foam plastic layer cast to the core.
In order to make such a spacer, a thin extruded or roll-formed core is supported in an elongated two-piece casting mold by a support rod. Curable foam plastic is cast into the annular space formed between the core and the mold. The foam is then cured and allowed to cool so that it shrinks to form a 25 to 150 mil thick layer around the core. The core itself is very thin, on the order of ten mils, and is made of an extruded or roll-formed material, either metal such as aluminum or steel, or some type of extrudable plastic such as PVC or phenylene oxide polymer. The foam casting material is a foam-in-place phenolic, polyester or polyurethane resin.
Such a spacer provides advantages due to the structural rigidity provided by the metal base. However, the spacer suffers from disadvantages in that the relatively thin coating of foam material may not serve as a thermally insulating bridge over the continuous metal tube. Further, such a spacer can be expensive to manufacture, because conventional injection molding techniques can be impractical to make such a thin hollow elongated body. In addition, vinyl spacers are generally poor sealants and are subject to mechanical failure.
U.S. Pat. No. 4,222,213 is an improvement over the spacer taught in the '905 patent. The spacer in the '213 patent includes a thin plastic insulating shape which is extruded and thereafter fitted by contact pressure or friction, over a portion of a conventional extruded or roll-formed metal spacer and has projecting contacts which abut the glass panes. The plastic insulating overlay can be formed over a conventional extruded aluminum spacer and from an extrudable thermoplastic resin. However, the force fit and the bimaterial construction of such a spacer can result in separation of the two components with changes in temperature due to the different thermal expansion coefficients of the metal and the plastic. This is undesirable.
Accordingly, a need has arisen to provide an insulating spacer which creates a thermally insulating bridge between spaced-apart panes in a multiple glass unit and which overcomes the above-noted drawbacks with conventional insulating spacers and those associated with conventional spacer manufacturing techniques.